It is well known in the art that the performance is greatly enhanced if linear precoding can be used at a transmitter side in wireless communication systems supporting multiple antenna transmissions. Such linear precoding has e.g. been implemented in the IEEE 802.16-2005 standard and in the 3GPP Rel.8 Long Term Evolution (LTE) standard.
To support precoding at the transmitter side, the receiver, sometimes also known as a User Equipment (UE) in the Downlink (DL), needs to feed back Channel State Information (CSI) about the multi-antenna channel between transmit and receive antennas. The CSI may consist of a representation of the actual multi-antenna channel, or alternatively a preferred precoding vector/matrix which the UE has determined based on measurements on the multi-antenna channel. In the latter case, the CSI is commonly referred to as a Precoding Matrix Indicator (PMI).
To reduce feedback overhead when signalling CSI reports, quantization is required so as to represent the CSI in a finite number of bits. As an example: the 3GPP LTE Rel.8 standard use a precoding matrix codebook consisting of 64 matrices and the UE feeds back the preferred precoding matrix using 6 information bits.
As mentioned above, a codebook of a finite number of matrices is commonly used to quantize the CSI in which case the CSI feedback is an index to a codebook that points out the matrix in the codebook that best represents the CSI. The index is then reported to the transmit node using for instance a string of binary bits.
As the channel is frequency and time selective by nature, a CSI report is only valid with reasonable accuracy up to some maximum bandwidth and for some maximum time. If the communication system wants to support transmission bandwidths using linear precoding for larger than this maximum bandwidth, feedback of multiple CSI reports is needed and further these CSI reports need to be repeated in time with appropriate intervals.
The bandwidth and time interval of each of these CSI reports are denoted as time-frequency granularity of the CSI, and if a codebook of matrices is used to quantize the CSI, one matrix is reported per time interval and frequency bandwidth.
To meet the high requirements on data throughput in future wireless communication systems, such as the 3GPP LTE-Advanced, an even larger number of transmitter and receiver antennas are envisioned. Since the dimensions of the multi-antenna channel thereby increases, the required CSI feedback overhead will increase further, thereby hampering the desired throughput increase.
Furthermore, when the number of antennas (or antenna elements) is increased, the physical dimensions of the transmitter and receivers will also increase, which is undesirable due to the larger area of e.g. a Base Station (BS) which will make it more vulnerable to environmental effects such as strong winds. Also, the architectural (visible) impact on buildings and the effect on landscape or cityscape should not be neglected in this context. To partly cope with the problem of larger antenna arrays dual polarized antenna elements is commonly assumed, since by utilizing two orthogonal polarizations of the electromagnetic field, one can effectively have two antennas in one. So, by utilizing polarized antennas, the total dimension of the antenna arrays is roughly halved.
Another obvious approach to make equipment with many antenna elements physically smaller is to reduce the spacing between the antenna elements. This will make signals received and transmitted more correlated (if they have the same polarization) and it is well known that the expected multiple antenna spatial multiplexing gain will be reduced. However, it is also known that correlated signals make very good and narrow beams, and the multiple antenna spatial multiplexing could then be used to transmit to users which are spatially separated. This is sometimes called Spatial Division Multiple Access (SDMA) or Multi-User MIMO (MU-MIMO). Hence, the drawback of lower per user throughput when using narrow spaced antennas elements can be compensated by transmitting to multiple users simultaneously, which will increase the total cell throughput (i.e. the sum of all users throughput in the cell).
Further, it is a known physical property that the channel emanating from antennas with orthogonal polarizations have close to independent fading. It is further known that channels emanating from closely spaced equally polarized antenna elements have correlated fading. Hence, for multi-antenna transmitters and receivers having a large number of antenna elements, compact antenna arrays which also utilize the polarization dimension is preferred. In this case, it is observed that among the antenna elements, the correlation between the radio channels between some pairs of the antenna elements is high, whereas the correlation for the radio channels between some other pairs of the antenna elements is low or even negligible. It is often said that two antennas are correlated meaning that the channel from the two antennas to any receiver antenna are correlated. This convention is used throughout the present disclosure.
In the 3GPP LTE standard specification TS 36.211, a codebook of 16 matrices is defined which facilitates feedback of dual polarized antenna arrays. Each matrix is thus indexed with a single 4 bit index. The feedback can be per sub-band which is a limited part of the total available bandwidth, or wideband which is the whole available bandwidth, i.e. the sum of all sub-bands. Hence, according to said specification a 4 bit PMI is fed back for each of N number of sub-bands, or for the wideband case. Therefore, 4*N feedback bits is needed when using the method in the TS 36.211 specification.